1. Field of the Invention and Contract Statement
The present invention relates to sample transfer pumps and in particular to pumps for sampling fluids at very small or very large pressures. The United States Government has rights in this invention pursuant to Contract No. DE-AC09-76SR00001 between the U.S. Department of Energy and E. I. DuPont de Nemours & Co.
2. Discussion of Background
There are a variety of fluid sample transfer pumps and, indeed, pumps of all sorts are well known. For sampling fluids at very large or very small pressures, typically a bladder is placed in an over-container which is evacuated to cause the sample to be drawn into the bladder. To expell the sample from the bladder, the over-container is refilled thereby expressing the sample from the bladder. See for example, the devices disclosed in U.S. Pat. No. 4,635,487 and 3,806,474.
Although pistons are efficient pumps, they are difficult to seal when the pressure differential across the piston is too great. Mechanical seals are used if standard O-rings will not hold the seal. A mechanical seal for sealing the edge of a moving piston against a cylinder wall presents significant, obvious design conflicts.
For very low pressures, the Toeppler pump has been used to transfer samples of gases in a high vacuum system. The Toeppler pump uses a reservoir of mercury to pump a gas from one container to another and can reduce the pressure in the container down to about 10.sup.-5 Torr. The Toeppler pump, even when automatically operated, is slow in moving a significant volume of gas.